It is known that during the extracorporeal treatment of blood, it is necessary to measure the pressure of either the blood or some other fluid included in the treatment. Normally, the fluid, such as blood, whose pressure is to be measured is connected up directly or via a filter to a pressure gauge wherein a positive, as well as negative, pressure can be measured. However, with such systems it has been found that coupling problems arise, since the elements carrying the fluid to be measured--for example, a flexible blood tube--are intended to be used only once, whereas the pressure gauge is intended to be used numerous times. Accordingly, problems arise with contamination and sterilization of the pressure gauge.
There is disclosed in U.S. Pat. No. 4,194,974 a system for measuring pressure which attempts to alleviate these coupling problems by allowing the pressure to be transferred mechanically from an elastic pressure pad to a pressure gauge. However, it has been found that the utilization of this system is not suitable for the measurement of negative pressures but, rather, is primarily used for checking that a maximum negative and/or positive pressure are not exceeded.
Another generally known method for measuring a negative pressure consists of comparing the negative pressure with a referenced pressure so as to arrive at a positive pressure difference which can be measured by any suitable method. This also has been found to be nonacceptable, since this procedure becomes relatively complicated because the absolute pressure is not measured directly.